1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to a method of manufacturing a negative electrode including graphite particles which can cause intercalation and de-intercalation of lithium ions.
2. Description of the Related Art
Secondary batteries using alkali metal such as lithium have attracted a lot of research efforts to comply with reduction in the size and power consumption of electronic equipments. The usage of alkali metal alone such as lithium for a negative electrode yields a problem that short circuit occurs within the battery by repeating operations of charge/discharge. Repetition of charge/discharge means that dissolution and precipitation of the alkali metal are effected repeatedly to result in growth of alkali metal dendrite on the surface of the negative electrode. This dendrite may become so great that it penetrates the separator between the negative electrode and the positive electrode to come into contact with the positive electrode, resulting in a short circuit.
The growth of dendrite can be suppressed to improve the charge/discharge cycle characteristics by using an alkali metal alloy as the negative electrode for a secondary battery instead of alkali metal. However, dendrite growth cannot be completely suppressed even in the case where an alkali metal alloy is used as the negative electrode. The possibility still remains of a short circuit within the battery.
Recently, a negative electrode of carbon or a conductive organic polymer material has been developed taking advantage of attraction and discharge of alkali metal ions instead of metal dissolution, deposition, diffusion in solid or the like carried out in the case of an alkali metal or an alloy thereof. In a negative electrode of carbon or conductive polymer material, dendrite generated in the case of a negative electrode of alkali metal or alloy thereof are not grown in principle. The problem of a short circuit caused by dendrite growth within a battery is circumvented.
Carbon is a preferable material as an electrode for a battery since it is chemically stable and can be doped with either an electron-donor element or an electron-acceptor element.
When carbon is used as an active material for a negative electrode, the amount of lithium that can be intercalated between layers of carbon is 1 lithium atom per 6 carbon atoms at maximum, i.e., LiC.sub.6 is the maximum. The theoretical electric capacity just by reaction between carbon and lithium is 372 mAh/g per unit weight of carbon at this upper limit.
Carbon can take various forms such as amorphous carbon to graphite. Sizes and arrangements of hexagonal nets of carbon atoms vary depending upon the starting materials and manufacturing processes. Usage of carbon materials without graphite as the active materials for negative electrodes are disclosed in, for example, Japanese Patent Laying-Open Nos. 62-90863, 62-122066, 63-213267, 1-204361, 2-82466, 3-252053, 3-285273, and 3-289068. None of the carbon materials disclosed in these documents can achieve the above-described theoretical capacity. Even carbon materials having a relatively large charge/discharge capacity exhibit linear change of a considerable gradient in potential during de-intercalation of lithium to be insufficient for the available voltage range in an actually assembled battery. It can be said that the carbon materials disclosed in the above cited documents are not satisfactory as the material for manufacturing a negative electrode in a battery with sufficient charge/discharge capacity.
Japanese Patent Laying-Open Nos. 4-112455, 4-115457, 4-115458, 4-237971, and 5-28996 disclose usage of graphite materials as the active materials for negative electrodes. The above-described theoretical capacity is achieved by none of these graphite electrodes. They are not satisfactory for use in manufacturing a battery with high capacity.
Japanese Patent Laying-Open No. 3-216960 discloses a secondary battery that allows a large current flow and improvement in cycle life time and safety by forming a lithium layer on the surface of a porous carbon material avoiding clogging of the pores. Another Japanese Patent Laying-Open No. 4-39864 discloses a secondary battery superior in charge/discharge cycle characteristics and improved in self-discharge characteristics with a great electrode capacity by using a negative electrode having a metal that can form an alloy with lithium or an alloy including lithium permeated into the pores of a carbon material. These batteries are disadvantageous in that the manufacturing process is complicated since lithium must be treated in an inactive atmosphere, resulting in increase in the cost thereof.
Although attempts have been made to improve the battery cycle characteristics and discharge efficiency after storage at a high temperature, for example, by using a negative electrode of a carbon material coated with metal such as nickel and copper as in Japanese Patent Laying-Open No. 4-184863, significant increase in the capacity of the negative electrode could not be expected.